In communication systems, transceivers often transmit and receive signals simultaneously in separated frequency bands. Typically, the amplitude of the transmit signal is higher than the receive signal causing problems because a portion of the transmission signal, which is referred to as echo, may leak from the transmit path into the receive path of the transceiver. As the frequency bands for transmitting and receiving are separated, filtering of the receive signal may eliminate at least a portion of the echo signals leaking from the transmit signals to the receive signals. However, even with separated frequency bands and filtering, echo signals may influence the receive signals at frequency close to the band separation mainly due to overlapping of the receive and transmit frequency bands caused for example by side bands etc. generated during the signal processing.
The echo signal adding to the receive signal may be eliminated or at least reduced by providing echo cancellation for the transceiver. In echo cancellation, the echo signal leaking from the transmit to the receive path is replicated based on the transmit signal and the replicated echo signal is subtracted from the receive signal to obtain a compensated receive signal.
Echo cancellation may for example be implemented by providing an echo path from the transmit path to the receive path. At the transmit side, the transmit signal or a copy of the transmit signal is provided to an echo cancellation filter. The filtering coefficients of the filter are selected or adjusted to obtain at the output of the filter a dublicate of the echo signal representing or closely representing the echo signal. The dublicate of the echo signal is then introduced to a subtracting node in the receive path for compensating the receive signal.
The echo cancellation path may be provided in pure digital domains of the transceiver, i.e. the input signal of the echo path is tapped from a digital domain of the transmit path and the dublicate of the echo signal is introduced to a digital domain of the receive path for compensating. The echo cancellation path can also be provided in pure analog domains, i.e. the input signal of the echo path is tapped from an analog domain of the transmit path and the echo signal dublicate is introduced to an analog domain of the receive path. Furthermore, the echo cancellation path may introduce the replica of the echo signal to a node in an analog domain, but the input signal for the analog signal may be tapped from a digital domain. A digital-to-analog converter (D/A converter) is provided in the echo cancellation path, typically after a digital echo cancellation filter. While in this case, the echo path comprises digital and analog components, it may be classified still as an analog compensation path because the compensation signal is provided in an analog part of the receive path.